The present invention relates generally to methods and apparatuses for measuring changes in length or position; more particularly it relates to reducing the data age differences between the multiple measurements of length and position.
The use of interferometry to measure changes in position, length, distance or optical length is well known, see for example xe2x80x9cRecent advances in displacement measuring interferometryxe2x80x9d N. Bobroff, Measurement Science and Technology, pp. 907-926, Vol. 4, No. 9, Sep. 1993 and U.S. Pat. No. 4,688,940 issued Aug. 25, 1987. A typical displacement measuring interferometer system consists of a frequency-stabilized light source, interferometer optics and measuring electronics. The interferometer optics split the laser light into a reference path and a measurement path, then recombine the light returning from the two paths and direct the recombined light to a photodiode where it produces an interference signal. A distance change of one wavelength in the measurement path relative to the reference path produces a phase change of 360 degrees in the interference signal. The measuring electronics measure and accumulate the change in phase and provide a position output for the application.
Many interferometer applications, such as step-and-scan photolithography tools used to manufacture integrated circuits, require measuring multiple axes of motion at high velocity and with high resolution. An advanced photolithography system may include measurement of eight axes. The accuracy requirements increase as the size of the features on the measured object decrease. Rapidly increasing accuracy demands and needs for determining the precise timing of multiple dynamic interferometric position measurements at higher accuracy have fueled numerous efforts to reduce and minimize the various sources of uncertainty that are inherent in currently known methods and apparatus.
Two forms of inherent uncertainty are called fixed delay and variable delay. Fixed delay arises from differences in cable lengths, optical path lengths, photoelectric detector delay, and phase meter offsets in interferometry systems, whereas circuit delay, i.e., group delay, (which varies with signal frequency) give rise to variable delay. The effects of these delays create differences in the data age of the interferometrically measured values, i.e., the elapsed time between the event representing the position measurement, and when the position data is available to the user. Compensating the data age by adjusting one or more of the delays is generally impractical.
Interferometric distance measuring, such as angle measurements, requires measuring two or more axes in order to provide the necessary information. To achieve full accuracy with dynamic multi-axis measurements, all measurements must have the same data age, which means that simultaneous measurement of each axis represents the same instant in time. Data age is defined as the time from when a change in interferometric position occurs to when the data representing the measured position is output. In a multi-axis dynamic system, when the system relies on position values from several different axes in motion, small differences in data age between axes can result in significant measurement errors.
Multiple-axis measurements are synchronized by a common reference provided to all phase meters and by a common sample control signal. The variation in data age among axes is due to differences in the measurement electronics, the measurement electrical and optical signal paths, the reference signal path, and the sample control signal. Differences in data age among axes must be minimized or eliminated.
In one aspect, the invention features a method for compensating data age in measurement signals from an interferometer. The method includes measuring a plurality of values of the measurement signal (e.g., amplitude, frequency, phase, time, or position); determining a dynamic data age adjustment value at each measured value of the measurement signal based on one or more processed values of the measurement signal obtained prior to that measured value; and adjusting a measured value of the measurement signal with a dynamic data age adjustment value to correct for data age. The processed values of the measurement signal such as phase, position, or frequency are derived from the measured value of the measurement signal.
In another aspect, the invention features a method for compensating data age in measurement signals from an interferometer by measuring a plurality of values of a measurement signal; and adjusting a measured value of the measurement signal with a data age adjuster to correct for data age of the measurement signal, wherein the data age adjuster includes a fractional data age adjuster having an interpolator, the interpolator utilizing a data age adjustment value to interpolate a value of the measurement signal between two adjacent values of the measurement signal. The method can include determining the data age adjustment value by determining a dynamic data age adjustment value at each measured value of the measurement signal based on one or more processed values of the measurement signal obtained prior to that measured value. The data age adjustment value can be constant for each of the values of the measurement signal. The data age adjuster can include an integer adjuster. The integer and fractional data adjusters can be separately located within an electronic architecture of the interferometer.
Embodiments of these aspects may include one or more of the following. The adjusted measurement signal is measured subsequent to the processed values of the measurement signal on which the dynamic data age adjustment is based. The method includes adjusting each of the plurality of measurement signals. The dynamic data age adjustment value corrects data age of the measurement signal in one or more of time, phase, position, and amplitude. The method includes adjusting a position value of one of the plurality of measurement signals to compensate for data age adjusting the time value of that measurement signal. Determining the dynamic data age adjustment value includes determining that value from a processed velocity value, a processed position value, a constant data age value, or combinations thereof. Determining the dynamic data age adjustment value includes using a processed velocity value and a processed position value derived from an earlier value of the measurement signal. Adjusting the measured value includes changing the measured value in integer units and fractional units based on the dynamic data age adjustment value. Changing the measured value in fractional units includes interpolating between two adjacent values of the measurement signal. The method includes digitizing the plurality of values of the measurement signal to produce a digitized representation of a plurality of values of an analog measurement signal.
In another aspect, the invention features an apparatus for compensating data age in measurement signals from an interferometer. The apparatus includes an electronic processing unit having a dynamic data age component configured to receive the plurality of values of the measurement signal, to determine a data age adjustment value based one or more processed values of the measurement signal, and to output the data age adjustment value; and a data age adjuster configured to receive the data age adjustment value and adjust a subsequent value of the measurement signal based on the data age adjustment value. The electronic processing unit is configured to receive a measurement signal from the interferometer, measure a plurality of values of the measurement signal, process values of the measurement signal, and output adjusted values of the measurement signal.
Embodiments of this aspect may include one or more of the following. The electronic processing unit includes a phase meter to determine the phase of the measurement signals. The phase meter determines the phase by discrete fourier transform. The data age adjuster is integrated into the phase meter. The electronic processing unit includes a phase connecting circuit configured to compensate phase ambiguity in the measurement signal values. The data age adjuster includes a fractional data age adjuster having an interpolator, the interpolator utilizing a data age adjustment value to interpolate a value of the measurement signal between two adjacent values of the measurement signal.
Embodiments of the invention may have one or more of the following advantages. The interferometric measuring system can adjust data age of the measured signal to account for group delay of the signal processing electronics, the path length delay, and delay due to fiber optic tolerances. The use of an interpolator for fine adjustment of data age can provide accuracy of 0.15 nm and a time resolution of about 30 ps. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.